The neurotransmitter(s) contained in baro- and chemoreceptor primary afferent nerves which project to the nucleus tractus solitarius is (are) unknown. One particularly promising agent to fill this role is the undecapeptide, Substance P (SP). Preliminary studies have demonstrated that SP is found at the site of projection of the peripheral as well as central processes of these bipolar neurons whose cell bodies are found in the nodose and petrosal ganglia. In addition, the SP in the nucleus tractus solitarius (NTS) is supplied in part by afferent input from baro- and chemoreceptor nerves. The aim of these studies is to demonstrate whether or not SP fulfills the additional criteria for an agent to be classified as a neurotransmitter in a particular system and to establish the role of the SP in the NTS in baro-and chemoreceptor reflexes. These problems will be approached by utilizing neurochemical and neuroanatomical methods as well as in vivo physiological and pharmacological studies. These studies will provide important information on the neurochemistry, neuroanatomy and functional role of Substance P. In addition, they will enhance our knowledge of CNS mechanisms for the control of the cardiovascular system.